1. Field of the Invention
The present invention relates to a frequency-variable PWM (Pulse Width Modulation) motor drive circuit capable of operating under different PWM frequencies. More particularly, the present invention relates to the frequency-variable PWM modulation motor drive circuit having a compensation unit connected between a drive IC member and a PWM converter circuit such that the PWM converter circuit is capable of operating under different PWM frequencies.
2. Description of the Related Art
Referring to FIG. 1, a conventional PWM motor includes a PWM motor drive circuit 1 electrically connected with a motor coil 2 so as to carry out alternatively magnetizing (energizing) the motor coil 2. The alternatively magnetized motor coil 2 can drive a motor rotor (not shown) to turn with respect to a motor stator (not shown) of the PWM motor. Typically, the PWM motor drive circuit 1 includes a drive IC member 10, a Hall IC member 11 and a PWM converter circuit 12. The drive IC member 10 electrically connects with the Hall IC member 11 so as to permit drive IC member 10 to receive rotor-detecting signals (i.e. rotational detecting signals) generated from the Hall IC member 11. However, the drive IC member 10 is designed to have a pin VTH which electrically connects with the PWM converter circuit 12. Correspondingly, the PWM converter circuit 12 has a PWM input pin 121 serving to introduce a PWM signal from an exterior system-(not shown). The PWM converter circuit 12 further has a transistor 122. The PWM signal is converted into a voltage signal by the transistor 122 of the PWM converter circuit 12, and then sent it to the pin VTH of the drive IC member 10 for controlling or adjusting a period of alternative magnetization of the motor coil 2. Accordingly, operational statuses of the motor are possessed of multi-speed modes in heat-dissipating operation by means of the PWM signal.
Generally, the motor divides the operational statuses into a high-speed mode (including full speed), a low-speed mode (excluding full or zero speed) and a stopping mode (zero speed). The drive IC member 10 can determine the operational statuses of the motor according to the input PWM signal such that the motor can be adjusted and changed in speeds to fulfill various system needs. For example, when a voltage of the pin VTH of the drive IC member 10 is higher than 3.6 volts, the drive IC member 10 controls the motor to operate at the stopping mode as well as zero rpm. Conversely, when the voltage of the pin VTH of the drive IC member 10 is lower than 2.0 volts, the drive IC member 10 controls the motor to operate at the high-speed mode as well as 6,000 rpm. If the voltage of the pin VTH of the drive IC member 10 is in the range of 2.0 volts to 3.6 volts, the drive IC member 10 controls the motor to operate at the low-speed mode as well as greater than zero rpm but lesser than 6,000 rpm.
Referring again to FIG. 1, the PWM motor drive circuit 1 is designed to have a capacitor 3 parallel-connected between the drive IC member 10 and the PWM converter circuit 12. Meanwhile, the capacitor 3 is designed to have a ground connection in place. In operation, the capacitor 3 is adapted to commutate a saw tooth wave input from the PWM converter circuit 12. However, the capacitor 3 of the PWM motor drive circuit 1 is so configured to stabilize the voltage of the pin VTH of the drive IC member 10. When the motor is actuated, the voltage of the pin VTH of the drive IC member 10 can determine and adjust the speed of the motor.
Referring to FIGS. 2A and 2B, the drive IC member 10 can control the motor to operate in the high-speed mode or the low-speed mode. In normal operation, the speed of the motor is operating at 2,000 rpm as well as low-speed mode when the voltage of the pin VTH of the drive IC member 10 is maintained at 3.0 volts (i.e. lesser than 3.6 volts but greater than 2.0 volts). But, in abnormal (high temperature) operation, the speed of the motor is operating at high-speed mode when the voltage of the pin VTH of the drive IC member 10 is dropped to zero volts (i.e. lesser than 2.0 volts). Still referring to FIGS. 2A and 2B, due to a ground connection, the voltage across the capacitor 3 is generally zero volts, as best shown in FIG. 2A, and the capacitor 3 can be charged by a voltage from a power supply when the motor is started. Inevitably, the voltage of the pin VTH of the drive IC member 10 is maintained at substantially zero volts. In this way, the drive IC member 10 can invariably control the motor to operate in the high-speed mode as long as the motor is started; namely, the speed of the motor is rapidly and shortly jumped to 6,000 rpm (i.e. full speed) from zero rpm, as best shown in FIG. 2B.
Referring back to FIGS. 1 and 2B, once started, the motor must inevitably enter the high-speed mode that must rapidly and shortly increase the speed of the motor. However, there is no greater amount of operational heat for dissipation. This results in the motor unnecessarily operating at full speed (i.e. top speed) that generates an increased amount of air noise and vibration. Furthermore, the motor occurs an increased amount of abrasion among motor components that may shorten the longevity of the motor.
Referring again to FIGS. 2A and 2B, the voltage across the capacitor 3 can reach 3.0 volts in the event after charging for a predetermined time. In this way, the voltage of the pin VTH of the drive IC member 10 is greater than 2.0 volts but lesser than 3.6 volts so that the drive IC member 10 terminates the motor to operate in the high-speed mode. Accordingly, the speed of the motor is dropped to a predetermined speed or a lower speed of 2,000 rpm.
However, ambient heat generated from a heat source is lower than a high temperature when the motor is started. Therefore, it is undesirable to permit the drive IC member 10 to increase the speed of the motor reaching 6,000 rpm in the high-speed mode that is unsuitable for the need of normal usage or an improper usage of the motor due to a waste of power consumption. Hence, there is a need for improving the motor to prevent entering the high-speed mode while starting.
In order to solve the motor to be unexpectedly operated at the high-speed mode while starting, an approach to this problem is disclosed in applicant's own U.S. patent application Ser. No. 11/274,417, the entire disclosure of which is incorporated herein by reference. In this approach, a capacitor is parallel connected between a pin VTH of the PWM drive IC member and PWM converter circuit, and the capacitor has an end further connecting with a power source. Accordingly, the voltage of the pin VTH of the PWM drive IC member 10 cannot drop to zero voltage in such a way as to prevent the motor from unexpectedly entering a high-speed mode while starting the motor.
Turning now to FIG. 3A, a pair of waveform diagrams show a PWM input terminal and a collector of a transistor Q1 of the conventional PWM motor drive circuit in FIG. 1 when PWM signals with 50%-duty cycle and frequency of 100 Hz are applied. With reference to FIGS. 1 and 3A, the PWM signals with 50% duty cycle and frequency of 100 Hz are supplied to the PWM input pin 121 of the PWM converter circuit 12. A waveform of the PWM signals of the PWM input pin 121 is identically corresponding to that of the collector of the transistor 122, but phases of them are completely complemented, as is demonstrated in observed experimental results in the study. That is to say, the motor speed can be almost constant as long as the frequencies of the PWM signals supplied to the motor are lower than 100 Hz.
Turning now to FIG. 3B, a pair of waveform diagrams show the PWM input terminal and the collector of the transistor Q1 of the conventional PWM motor drive circuit in FIG. 1 when different PWM signals with 50% duty cycle and frequency of 100 KHz are selectively applied. In comparison with a waveform of the PWM signals input to the PWM input pin 121, a waveform of the collector of the transistor 122 is distorted greatly and the ratio of a peak to a wavelength of this waveform is specifically reduced. This results in an incorrect motor speed relative to a predetermined motor speed due to duty cycle less than 50%. The PWM signals possess the same of 50% duty cycle but the frequencies has changed to 100 KHz. Similarly, if other higher frequencies of the PWM signals are applied to the PWM input pin 121 of the PWM converter circuit 12, the motor speeds can be shifted due to a reduction of duty cycle, and cannot be consistent with predetermined motor speeds in relation to selected duty cycles. Frankly, the conventional PWM converter circuit 12 is only suitable for applying in the frequencies of the PWM signals lower than 100 Hz, and is unsuitable for applying in the frequencies higher than 100 Hz, other higher frequencies, or the frequency of 100 KHz.
As is described in greater detail below, the present invention intends to provide a frequency-variable PWM motor drive circuit capable of operating under different PWM frequencies, wherein a compensation unit connects between a drive IC member and a PWM converter circuit. The compensation unit can compensate distortions of a waveform supplied from the PWM converter circuit due to changes in frequencies of PWM signals. Accordingly, the frequency-variable PWM motor drive circuit can be applied in various frequencies of the PWM signals in such a way as to mitigate and overcome the above problem.